1. Field of the Invention
This invention relates to the separation of materials, produced for example by a mining operation, into fractions of different densities.
2. Description of the Prior Art
In for example the mining of coal, a mixture of coal and shale is produced from the face being mined, and it is necessary to separate the coal from the shale by the use of automatic machinery. Conventionally used for this operation is a wash box (hereinafter referred to as "a wash box of the kind specified") essentially in the form of a vessel divided vertically into a stratification compartment and a reject compartment, with a perforate grid plate extending across upper parts of the compartments. The vessel is filled to a level above the grid plate with a liquid, such as water, and the raw material is deposited on the grid plate at one side, flow of water across the grid plate tending to carry the material across the grid plate.
Means is provided to cause generally vertical pulsations in the liquid in the two compartments, disturbing the raw material on the grid plate, causing the heavy material (the shale) to sink to the bottom adjacent to the grid plate, and the lighter material to rise to the top. Thus, as the raw material moves across the grid plate, and in particular across that part of the grid plate extending over the stratification compartment, the raw material is stratified. The heavier material adjacent to the grid plate fails from the grid plate into a reject extraction chamber, and the light material passes over a gate or sill (located a short distance above the grid plate) and from the wash box through a primary outlet.
It is desirable in wash boxes of the kind specified to be able to control the operation of the wash box as accurately as possible, and preferably by means which permits the utilisation of automatically operating mechanisms.
For example, in such wash boxes, it is necessary to control the thickness of the layer of the heavier material on the grid plate in the stratification compartment so as to ensure reliable separation of materials. If the layer of the heavy material becomes too thick, particles of the heavier fraction will pass out with the lighter material over the gate or sill. Conversely, if the layer of the heavier material is too thin, particles of the lighter fraction will be discharged into the reject extraction chamber with the heavier material.
Thus, it has been proposed in a wash box of the kind specified to sense the thickness of the layer of the heavier material in the stratification compartment and, when a reduction in the thickness of this layer is required, to cause pulsations in the liquid in the reject compartment, or to increase the amplitude of the pulsations being applied thereto, so that the particles of heavier material will pass over the grid plate and fall therefrom into the reject chamber more readily.
Conventionally the thickness of the layer of heavier material on the grid plate has been measured by the use of a float which rests on the top surface of the heavier material, and the position of the float has been used to control the pulsations applied to the reject compartment.
Additionally, since the pressure which is generated in the stratification compartment will be dependent upon the weight of raw material resting on the grid plate, proposals have also been made to connect a vertical tube to the stratification compartment from beneath the grid plate, into which liquid from beneath the grid plate flows, and to sense the mean or average position of the surface of the liquid within this tube by the use of mechanical means which includes a float in the tube, thereby providing an indication of the thickness of the layer of the heavier material. Thus, the position of the float may be used to control the amplitude of the pulsations applied to the reject compartment.
Alternatively, electrodes within such a tube have been used to sense when the level of the liquid within the tube rises to a predetermined point (corresponding to an undesirably high thickness of heavier material on the grid plate) to cause an increase in the amplitude of the pulsations applied to the reject compartment, or to sense when the level of liquid within the tube falls below a predetermined point (corresponding to an undesirably small thickness of the heavier material on the grid plate) to cause a decrease in the amplitude of the pulsations applied to the reject compartment.
Whereas all previous suggestions have been to varying extents satisfactory in the past, they do not readily lend themselves to the solution of problems now being encountered in the field of materials separation, particularly as it concerns the mining of coal.
Thus, when coal was extracted manually from the face being worked, the raw material would contain a relatively small proportion of reject materials of a heavier density, for example, ten percent of the raw material would be shale. However, even with the use of coal cutting machinery, until recently coal seams which have been worked have been relatively thick, and of a reliable nature. Thus, it was possible to use the coal cutting machine in a manner such that the proportion of reject materials of heavier density cut from the face with the coal was still relatively small, although usually higher than the figure which was obtained with manual extraction.
However, there is in present times a tendency to work seams of decreasing reliability, involving at times the cutting of predominantly reject material, and to work seams of shallower depth, and/or over and undercutting to an extent which ensures that all the coal is removed, despite the increase in reject material which will necessarily be produced at the same time.
Thus, not only is the reject content of the raw material becoming increasingly higher (often up to seventy percent) but also the proportion of reject material may vary considerably over relatively short periods of time.
Present techniques for controlling the operation of wash boxes of the kind specified have heretofore become insufficiently accurate, and have produced difficulty in ensuring that a minimum of reject material is included in the coal, and in ensuring that a minimum of coal is included in the reject material.